Why not just type (Invisible ink) into Google? I just did and all sorts of sources pop up, including suggestions like how to use lemon juice to make invisible ink! Also pens for sale loaded with invisible ink, and/or the inks themselves!

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When I was young, there was something about lemon juice and a lighter, but can't quite remember how it goes...

As a kid I have also played with invisible ink. Ordinary household vinegar works well as an invisible ink. You develop the secret document by ironing it with an iron set to high temperature or "cotton" setting. The acid in the vinegar or lemon juice changes the paper, so it is more easily scorched, so your developed secret writing will appear brown.

I'm looking for 'watermark' and or invisible ink. That only shows up when held to a light source. For example say I printed trees on a white sheet of paper, and when I held said image up to a light source, a sun would appear, or an image that was otherwise invisible. I've looked around and also herd things about photo emulsion (have no idea how this relates to watermarks), screenprinting, and stamps with regards to watermarks. Any and all help/suggestions appreciated.

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You can find this ink on Securetechink.com
here is a video demonstration youtube.com/watch?v=91fVqI_SrCg
Its what you are looking for, can only be seen in UV light.